would you like us to guess? Problem is we really need to see your watch or if we knew the exact model number may be it has a probably unique for that model number not that I'm aware of such a thing but model number be helpful. I don't suppose you went swimming with the watch and it's filled with rust that would be a reason. But until someone takes the back off and disassembles the watch everything will be a guess and that might not necessarily be a good guess.
In an Omega 564 there are 56 parts involved in the winding mechanism. 35-plus of them can contribute to a problem. Sticky old lubricants (very common), broken mainspring (common), rust (possible), broken jewels (not uncommon), long synthetic sweater fibers wrapped around a wheel (more common than you'd think), etc. As John said, without tearing the watch down there's no way to guess.
I have one Constellation waiting in line right now that's showing that behavior. I returned another Constellation earlier this week that would auto-wind but wouldn't stem-wind - it turned out that the crown wasn't pushed all the way in.